Pressure switches have been used in a large variety of applications where the pressure of a fluid is used to alter the condition of an electrical circuit. Once particularly familiar application is in connection with air compressors. In such a case, a positive displacement machine is frequently driven by an electrical motor. The positive displacement machine draws air from the ambient and compresses it to a higher pressure. The compressed air is then discharged into a reservoir for storage for subsequent use.
As is well known, the reservoir will typically have pressure limitations. That is to say, if the internal pressure of the fluid stored within the reservoir exceeds some predetermined value, there is the possibility that the reservoir may rupture.
To prevent such from happening, the compressor system is typically provided with a pressure switch. When the pressure in the reservoir reaches some predetermined value, the pressure switch opens to open the circuit for the electrical motor driving the positive displacement machine. This, in turn, means that the compression part of the operating cycle will terminate and there will be no further increase in pressure within the reservoir as no further air is being delivered thereto by the now inoperative compressor.
As the pressurized fluid within the reservoir is utilized, the pressure in the reservoir will drop. At some predetermined pressure, typically somewhat below the shut off value mentioned previously, the pressure switch will close to again energize the electrical motor for the positive displacement machine. This, in turn, will cause the reservoir to be refilled until the first predetermined pressure is again reached, at which time the pressure switch will open to stop further operation of the electrical motor.
Similar systems are employed with fluids other than gaseous fluids as, for example, in water pumping systems. For example, in a typical home water system supplied by well water, a submersible pump is operative to elevate water to a sealed reservoir or tank. Because the tank is sealed, as it fills, the pressure of air above the water in the tank increases. This pressure head is used to drive water from the tank into the distribution system within the home when faucets or valves are opened. Check valves are utilized to prevent such pressure from driving the water back into the well.
Quite typically, the pressure within the reservoir is taken as a measure of the degree of filling thereof. Most usually, pressure is monitored at a tank outlet which will be below the level of water in the reservoir by means of a water pressure sensing switch, which in turn is operative to initiate operation of the pump when the pressure is low and to halt operation of the pump when the pressure is raised to a level indicating that the reservoir has been sufficiently filled.
Returning the matter of air compressing systems, those skilled in the art will readily appreciate that when the positive displacement machine is shut down by the pressure switch, there will be a relatively high residual pressure in the conduit leading from the positive displacement machine to the reservoir, as well as in the positive displacement machine itself. In order to restart the air compressor, it is necessary that the same be powered sufficiently so as to overcome the resistance provided by the residual high pressure. This frequently would require an overly large motor solely for starting purposes where a much smaller motor would be capable of driving the machine after it has been started. This in turn can mean higher levels of energy consumption, as well as a higher initial capital cost because the need for an overly large motor.
To avoid this, it has been conventional in the air compressor industry to provide a so called "unloader" valve. An unloader valve is operative, upon cessation of operation of the positive displacement machine, to vent the machine, but not the reservoir, to the atmosphere so that internal pressure within the machine is atmospheric pressure as opposed to the much higher pressure in the reservoir. Thus, when the machine is restarted, it need only overcome the resistance of friction, allowing the use of a relatively small motor. In the usual case, pressure switches are configured so that as they revert from a closed or electrically making position to an open or electrically breaking position, they also operate the unloader valve.
One difficulty faced by the manufacturer of pressure switches is the fact that not all of its customers will have the same configuration of compressor system components. That in turn means that some customers may wish the compressor connection port on the unloader valve to be opening upwardly, some to the right side, some to the left side, etc. To manufacture a wholly different pressure switch for each such situation is obviously undesirable because of the increased costs involved in making three different switches, for example, rather than one.
Pressure switches also customarily include a manual override with a manual operator which can be manipulated by the user to initiate operation of the motor being controlled by the switch or to halt its operation. Typically, these operators are in the form of a lever which protrudes from one side of the switch. Again, depending upon the configuration of system components, one system manufacturer may want the manual operator to protrude from one side of the switch while another system manufacturer will want the operator to protrude from another side of the switch. Again, two different switch configurations are called for and to manufacture two separate switches for the purpose would again be unduly expensive and inefficient.
Still, another problem that is commonly encountered in the operation of systems of this type may be chattering of the electrical contacts relative to one another when in the electrical making or closed position. Chattering causes premature wear and can cause pitting and even arcing difficulties in severe situations. In the usual case, chattering results from vibration imparted to the pressure switch during operation of the positive displacement machine which frequently includes a piston. Not infrequently, the plane of closure of the contacts will be either parallel to the base of the switch, or at right angles with respect thereto. The switch itself will be mounted in a system such that its base will be parallel to or at right angle to the direction of reciprocation of the piston in the positive displacement machine. In such a situation, contact chatter is at its worst.
Those skilled in the art will also recognize that pressure switches typically have a differential mechanism which has the effect of setting the pressure at which the switch contacts will open over that at which they will close. Typically, this mechanism is utilized to set the differential between the pressure at which the compressor is turned off and the second and lower pressure whereat it will again be turned on to replenish the reservoir. As this mechanism sets the pressure level based on the highest pressure encountered in the cycle, adjustment of it may result in a change in the pressure at which cut out occurs. More desirably, the differential should be adjusted based on the so called "cut in pressure" whereas operation of the positive displacement machine is resumed in response to partial depletion of the reservoir. In this way, a desired differential can be achieved without altering, in any way, the high pressure or cut out pressure of the system.
Also as is well known, many pressure switches include at least two levers mechanically interposed between a pressure sensing device such as a diaphragm and the electrical contacts that are to be opened or closed. In the usual case, the levers are mechanically coupled via an over center mechanism to provide snap action opening and closing of the contacts to minimize arcing and the resulting pitting of the contacts. Needless to say, the pressure switch should have a suitable frame for pivotally mounting these levers so that the same will not bind or otherwise operate unreliably. At the same time, it is desirable that the manufacturing cost of a switch be as low as possible without sacrificing reliability. Though precision techniques are not favored because of the costs thereof, at the same time they may be required in some degree in order to achieve the desire of reliability. Obviously, inexpensive fabrication techniques that can be employed without sacrificing reliability would be highly desirable.
The present invention is directed to overcoming one or more of the above problems.